1. Field of the Invention
This invention relates to an improved drum brake and, more specifically, to such a brake having a pair of floating brake shoes, each of which has a lining including a useable layer thereof which is thicker at the trailing end during normal drum rotation than it is at the leading end.
2. Description of the Prior Art
Drum brakes have heretofore been extensively used for the wheels which are rotatably mounted on the axles of heavy duty vehicles. The drums are mounted for rotation with the wheels and a brake spider is mounted on the axle to include some form of actuation means for a pair of arcuate brake shoes to cause them to be brought into frictional engagement with the opposite sides of the interior surface of the drum. Many types of actuation means and many methods of mounting the brake shoes have been successfully utilized in the past.
Because it is desirable for the brake shoe lining to be brought into proper and full contact with the interior of the drum throughout its life, one type of drum brake includes brake shoes which are capable of "floating" during brake operation. Neither end of a floating brake shoe is mounted on a pivot pin or other means which would restrict it movement throughout the life of the brake shoe lining. Although one prior art drum brake utilizing such floating brake shoes has been satisfactorily employed in heavy duty trucks and trailers, it has been found that its use in bus or coach installations has resulted in uneven wear of the useable lining thereof even though the braking function has worked satisfactorily throughout the use of the lining. The useable lining was designed to be thicker in the intermediate region of the shoe rather than at its ends. Such a lining is well known in the brake art as, for example, can be seen in U.S. Pat. Nos. 2,167,607; 2,213,003 and 2,381,737.
However, although there was considerable wear in the intermediate region of these prior art brake shoes after repeated braking in a normal direction of rotation of the wheel and drum, the useable layer was found to be initially worn away at the trailing end of the shoe with very little wear at the leading end. Apparently, the multiple and frequent braking of such coaches has resulted in a combination of variables which either did not exist in the heavy duty truck and trailer installations or which, if they existed, did so in such an infrequent manner that a similar brake lining wear pattern was not apparent. In any case, although this prior art drum brake efficiently and reliably produces braking for the coaches, the uneven wear on the lining has significantly decreased the effective life of the lining and has increased maintenance problems associated with its replacement.